Causes

Note The memory usage of ECS instances does not indicate whether requests are evenly distributed.

Session persistence is enabled.

If session persistence is enabled, it will cause traffic imbalance when few clients
are accessing the Server Load Balancer (SLB) instance. This is especially common when
a small number of clients are used to test the SLB instance. For example, session
persistence (based on source IP addresses) is enabled for a TCP listener and a client
is used to test the load balancing service.

The ECS instance status is abnormal.

Backend servers with abnormal heath status can also lead to an imbalance especially
during a stress test. If the health check for a backend ECS instance fails or the
health status of a backend ECS instance changes frequently, this will cause an imbalance.

TCP Keepalive is enabled.

When some backend ECS instances enable TCP Keepalive and others do not, the connections
will accumulate on the ECS instances with TCP Keepalive enabled. This scenario will
cause an imbalance.

Troubleshooting

Check whether the weights of backend ECS instances are the same.

Check whether health checks of backend ECS instances fail or whether the health status
is unstable in a specified period. Check whether the health check is correctly configured
with the status code.

Check whether both the WLC scheduling algorithm and session persistence are enabled.
If so, change the scheduling algorithm to WRR.